Polymer
Brushes Patterned with Micrometer-Scale Chemical
Gradients Using Laminar Co-Flow
- Publication date
- Publisher
Abstract
We
present a facile microfluidic method for forming narrow chemical
gradients in polymer brushes. Co-flow of an alkylating agent solution
and a neat solvent in a microfluidic channel forms a diffusion-driven
concentration gradient, and thus a gradient in reaction rate at the
interface of the two flows, leading to a quaternization gradient in
the underlying poly(2-(dimethylamino)ethyl methacrylate) polymer brush.
The spatial distribution of the quaternized polymer brush is characterized
by confocal Raman microscopy. The quaternization gradient length in
the polymer brush can be varied with the injection flow rate and the
distance from the co-flow junction. A chemical gradient in the polymer
brush as narrow as 5 μm was created by controlling these parameters.
The chemical gradient by laminar co-flow is compared with numerical
calculations that include only one adjustable parameter: the reaction
rate constant of the polymer brush quaternization. The calculated
chemical gradient agrees with the experimental data, which validates
the numerical procedures established in this study. Flow of multiple
laminar streams of reactive agent solutions enables single-run fabrication
of brush gradients with more than one chemical property. As one example,
four laminar streamsneat solvent/benzyl bromide solution/propargyl
bromide solution/neat solventgenerate multistep gradients
of aromatic and alkyne groups. Because the alkyne functional group
is a click-reaction available site, the alkyne gradient could allow
small gradient formation with a wide variety of chemical properties
in a polymer brush